Attention-Deficit/Hyperactivity Disorder (ADHD) is the most common neurobehavioral disorder diagnosed in childhood and adolescence and one of the most prevalent chronic health problems afflicting school-age children in the US. One of the cornerstone treatments of the disorder is pharmacotherapy with formulations of the psychostimulants (STIMs) methylphenidate (MPH) or amphetamine (AMP) which represent by far the most common treatments in developing children and adolescents. The use of STIMs has accelerated rapidly in the past decade with prescriptive use of these agents at an all time high. Significant limitation exist regarding long-term use of these agents, including non-response or intolerable side effects in up to 30 percent of patients and the dsk of drug-drug interactions as part of increasingly complex drug regimens. Additionally, unlike many pediatric therapeutic agents, weight-based dosing recommendations have not proven clinically useful and therapeutic drug monitoring is not predictive of response. As with many CNS drugs, significant interindividual variability in the disposition of STIMs, therapeutic response, and side effects is common. In this R21 application, animal studies are proposed to support a long-range goal to improve the outcome of the treatment of ADHD with STIMs by identifying pharmacogenetic biomarkers. This application involves translational research addressing in part, the specific needs outlined in the program announcement, PA-03-113 (Developmental Psychopharmacology). Although the STIM agents have been in clinical use for over half of a century, and their metabolism and disposition are reasonably well defined, essentially no data exist on the role of drug transporters in their disposition and clinical effects. The most prominent gene of the ABC cassette family of transporters, ABCB1 (also known as MDR1), is highly polymorphic and encodes for P-glycoprotein (PGP). PGP, as an effiux transporter, provides a protective role for major organs by limitingcellular uptake of its substrates by excreting them into bile, urine, the intestinal lumen, and limiting their accumulation in brain. We hypothesize that PGP is an important determinant of brain access and pharmacologic effects of STIM medications. Futhermore, inhibitionor induction of PGP in the endothelial cells at the blood brain barrier by specific therapeutic agents should increase or decrease, respectively, the brain access and effects of STIMs-possibly in a stereoselective manner, and without altering their systemic exposure significantly. The specific aims are (i) to conduct stereospecific studies assessing whether MPH or AMP are PGP substrates using transgenic mice; (ii) to determine the effects of PGP inhibition on plasma and tissue concentrations of STIMs; (iii) to obtain pharmacodynamic correlates (e.g. locomotor behavior) of differential brain penetration of STIMs. The proposed data could support future human trials to improve ADHD therapy by genotyping patients to determine PGP status prior to drug selection and dosage, predicting and avoiding deleterious drug-drug interactions, and to explore the use of PGP modulation to influence brain penetration of STIMs. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]